1. Field of the Invention
The present invention relates to high color fidelity thin-film multilayer systems for use in head-up displays, and more particularly to a combiner of head-up displays, wherein the reflectance is independent of variance in wavelength throughout the visible light range.
2. Description of Prior Art
In order to prevent a driver from becoming unaware of traffic condition while looking at a dashboard, a head-up display is normally provided on the windshield of a vehicle to inform the driver of the conditions of the vehicle while providing a view outside the vehicle. Moreover, due to the increase of information provided to aid driving, such as traffic conditions and road maps, multi-color or full-color display facilitates identification of different information. Hence, the light source for a head-up display gradually moves toward full-color LCDs instead of conventional LEDs. Furthermore, the thin-film multilayer combiner coating system formed on the windshield should be operable at various wavelengths for display.
In the prior art, some methods for manufacturing thin-film multilayer systems have been developed. U.S. Pat. No. 4,968,117 disclosed a holographic multilayer system but this is only suitable for use with a monochromatic light source. U.S. Pat. No. 5,050,966 discloses a thin-film multilayer system utilizing cholesteric LC, which is suitable for use with a multicolor light source. However, it is very difficult to provide a full-color display because the LC components have to be adjusted corresponding to desired wavelengths. U.S. Pat. No. 5,162,928 provides a frequency-doubled multilayer system, which can only be used with a monochromatic light source. U.S. Pat. No. 5,496,621 provides an optical thin-film multilayer system that can be designed to meet the requirements of the color spectrum. It can provide a full-color display if appropriately designed.
In the prior art, metal film such as Al or Ag was used to fabricate an optical thin-film multilayer system to obtain reflectance less sensitive to the variance of wavelength and polarization. Yet the metal film possesses high absorptance, such that the transmittance cannot reach the maximum, that is, the value of (1-reflectance). On the other hand, as in U.S. Pat. No. 5,496,621, the thin-film multilayer system includes two oxide layers, which do not absorb light, and thus tend to vary the color being displayed with the variance of incident angle. Furthermore, the reflected light from the thin-film multilayer system for displaying the vehicle's conditions is not polarized, and thus cannot be increased without sacrificing the high transmittance of the light from outside the vehicle.